Typically, a motor control system, such as in an electric power steering (EPS) system, controls an electric motor by applying voltage to the electric motor. For example, an EPS system uses the electric motor to provide steering assist to an operator of a steering wheel of a vehicle using a method of torque control. When using a permanent magnet synchronous machine (PMSM), field oriented control (FOC) is utilized to allow the alternating current (AC) three-phase motor voltage and current signals to be transformed into a synchronously rotating reference frame, commonly referred to as the d/q axis reference frame. In a d/q axis reference frame, the motor voltages and currents become direct current (DC) quantities. The FOC torque control technique is commonly implemented either using feedforward methods of control or a closed loop current feedback control.
When the FOC torque control is implemented through a closed loop current control, the implementation employs high performance current controllers that minimize the error between commanded and measured currents to achieve perfect current tracking. Thus, current control requires the motor currents to be measured, which is typically achieved by measuring the phase currents of the electric machine, which are then transformed into the synchronous frame via the Park Transform to perform the control in the synchronous reference frame.
Accordingly, it is desirable to have motor control systems to have fault tolerant current measurement strategies in case of a fault/loss of one or more current measurement sensors, or other equipment used for measuring the current values. However, feedforward current control, which is the mode of operation of motor control systems with current sensing failures, is not as consistent and accurate as feedback current control, particularly due to its sensitivity to motor control parameter estimates. Thus, under feedforward control operation, it is desirable to implement control techniques that provide have higher accuracy and consistency of the feedforward control behavior.